Thermosetting resin compositions are generally superior to thermoplastic resins in heat resistance and also in other characteristics, such as electrical insulation and dimensional stability. For this reason, thermosetting resin compositions are widely used as a material for the substrate, which forms the insulating layer of a printed circuit board.
When used as a substrate material, thermosetting resin compositions are required to be excellent not only in heat resistance but also in flame retardance after hardened. These compositions, however, are mainly composed of a resin component, which is an organic material and therefore is comparatively low in flame retardance. A general approach to improving the flame retardance is to add a flame retardant to the compositions.
Widely used examples of the flame retardant are halogenated flame retardants, such as bromine-containing organic compounds and brominated epoxy resins. These flame retardants can be used in comparatively small amounts to provide thermosetting resin compositions with flame retardance while keeping their electrical and heat-resistance properties.
Substrate materials containing a halogenated flame retardant, however, may generate hydrogen halide or other harmful substances when they are burned. The substances are expected to have an adverse effect on the human body and the natural environment, and therefore, the substrate materials are required to be halogen-free. In recent years, it has been proposed to use, as substrate materials, various resin compositions that contain phosphorus-containing compounds in place of the halogenated flame retardants.
For example, PTLs 1 to 5 disclose, as substrate materials, resin compositions made halogen-free by adding phosphorus-containing compounds.
PTL 1 shows a powdered epoxy resin composition containing an epoxy resin, a phenol resin, a phosphor compound, and an inorganic filler surface-treated with a coupling agent.
PTL 2 shows an epoxy resin composition used for an electrical laminate. The composition contains, as essential components, an epoxy resin and a hardener, which is a predetermined modified phenol resin. The composition further contains a phosphorus flame retardant.
PTL 3 shows an epoxy resin composition containing an epoxy compound, a polyphenylene ether, a cyanate ester compound, an organometallic salt, and a phosphorus flame retardant. The epoxy compound has a number-average molecular weight of not more than 1000, at least two epoxy groups per molecule, and no halogen atom. The polyphenylene ether has a number-average molecular weight of not more than 5000.
PTL 4 shows a resin composition containing a low-molecular-weight polyphenylene ether, an epoxy compound, a cyanate compound, a phosphinate-based flame retardant, and a polyphosphate-based flame retardant having a triazine structure. The polyphenylene ether has a number-average molecular weight of 500 to 2000, and an average of 1.5 to 2 hydroxyl groups per molecule. The epoxy compound has an average of at least two epoxy groups per molecule. The cyanate compound has an average of at least two cyanate groups per molecule.
PTL 5 shows a polyphenylene ether resin composition containing a polyphenylene ether resin having a predetermined terminal structure, a cross-linking hardener, a phosphinate-based flame retardant, and a curing catalyst.